Voltage regulator circuits or voltage regulators are widely used in many applications to provide a nearly constant output voltage at a desired level that is substantially independent of a poorly specified and often fluctuating input voltage and output conditions (i.e., variation in a load current).
One type of voltage regulator is a replica voltage regulator. In a replica voltage regulator a voltage established in one portion or one leg of a circuit is replicated in another leg or portion of the circuit, typically by larger sized devices, to provide a desired load or output voltage. The output voltage is regulated by having it track the voltage in the first leg or portion as closely as possible.
An example of an output stage of a replica voltage regulator architecture for which a circuit and method of the present invention is particularly useful is shown in FIG. 1. Referring to FIG. 1A, the voltage regulator 100 includes a reference leg 102 coupled between a voltage source (Vpwr) and ground 104, and an output leg 106 coupled between Vpwr and an output node (Vout). The reference leg 102 includes a first transistor 108 connected as a source follower (SF) and including a gate node (Vgate) coupled to and controlled by for example an operational amplifier or a charge pump (not shown) in the voltage regulator 100, and an output node (Vsource) coupled to ground 104 through a series resistor network 110. The output leg 106 includes a second larger transistor 112, also connected as a source follower and controlled by the gate node (Vgate) of the first transistor 108. The voltage regulator 100 further includes a small feedback resistor (Rf 114) coupling the output nodes of the first transistor 108 (Vsource) and the second transistor 112 (Vout) to improve the accuracy and stability of the regulator. The first and the second transistors 108, 112 are selected so that the output voltage Vout is a replica of the Vsource voltage. A ratio between resistors R1 and R2 in the series resistor network 110 is selected so that Vsource is equal to the desired target voltage—that is it is the same as the desired Vout.
In normal operation Vpwr is greater than Vout and current flows through the reference leg 102, indicated by arrows 116, generating the desired target voltage at the output node of the first transistor 108 (Vsource), which is then replicated at the output node of the second transistor 112 (Vout). Current, indicated by arrows 118 and 120, flows from the sources (Vsource and Vout) of the first and the second transistors 108, 112 to the output node (Vout) of the voltage regulator 100.
Although the above described circuit provides a simple architecture that occupies a small area on a silicon die or substrate, it is not wholly satisfactory for a number of reasons. In particular, referring to FIG. 1B, when Vpwr goes lower than the output voltage (Vout) of the voltage regulator 100, the source potential (Vsource and Vout) of the first and the second transistors 108, 112 becomes higher than the drain potential (Vpwr) causing reverse currents, indicated by arrows 122 and 124, to flow from the source to the drain of the source follower transistors. Yet another leakage path allows a reverse current, indicated by arrow 126, to flow from Vout through the feedback resistor (Rf) 114 and the resistor network 110. The sum of these reverse currents can be substantial, on the order of several milliamps (mA), and can induce a drop or droop in the output voltage (Vout) and will quickly discharge batteries in battery operated devices.
Accordingly, there is a need for a circuit and method that substantially prevents or interrupts a reverse current flow into a voltage regulator and the resultant droop in output voltage when a voltage of the voltage source (Vpwr) drops below a voltage at the output of the voltage regulator (Vout). It is further desirable that the circuit and method substantially not effect performance of the voltage regulator under normal operating conditions, i.e., when Vpwr is greater than Vout.